The present invention generally relates to optical communication systems, and more particularly to an optical communication system which transmits an overhead signal together with a main signal in working and protection optical transmission lines.
Recently, in networks for making large-capacity transmissions using optical fibers, a configuration shown in FIG. 1 is usually employed in order to improve the reliability of the network. More particularly, a working transmission line 3 and a protection transmission line 4 are provided between optical terminal apparatuses 1 and 2.
In order to enable supervision and control of an alarm as well as modifications of various settings when making a communication between the optical terminal apparatuses 1 and 2 shown in FIG. 1, it has become popular to multiplex an overhead signal to a main signal and to make the communication between the optical terminal apparatuses 1 and 2 using high-level data link control (HDLC) procedures or the like.
FIG. 2 shows an example of the construction of the optical terminal apparatus 1 or 2 shown in FIG. 1. For the sake of convenience, it is assumed that FIG. 2 shows the construction of the optical terminal apparatus 1. The optical terminal apparatus 1 shown in FIG. 2 includes a multiplexer 11, a demultiplexer 12, a multiplexer 13, a demultiplexer 14, a switching circuit 15 and an overhead input/output circuit 20 which are connected as shown.
The multiplexer 11 multiplexes the main signals from a channel part (not shown) of the optical terminal apparatus 1, and transmits a multiplexed signal to the working transmission line 3. On the other hand, the demultiplexer 12 demultiplexes a received signal that is obtained via the working transmission line 3, and supplies demultiplexed signals to the channel part via the switching circuit 15.
The multiplexer 13 receives in parallel the main signals supplied to the multiplexer 11, and multiplexes the main signals. A multiplexed signal is transmitted from the multiplexer 13 to the protection transmission line 4. On the other hand, the demultiplexer 14 demultiplexes a received signal that is obtained via the protection transmission line 4, and supplies demultiplexed signals to the channel part via the switching circuit 15.
The overhead input/output circuit 20 generates an overhead signal OH. This overhead signal OH is supplied in common to the multiplexers 11 and 13, so that the overhead signal OH is multiplexed with the main signals in the multiplexers 11 and 13. The overhead input/output circuit 20 also receives via the switching circuit 15 the overhead signal OH which is separated from the main signals in the demultiplexers 12 and 14.
The switching circuit 15 receives a transmission line supervision alarm signal ALM1 which is separated in the demultiplexers 12 and 14, and carries out a switching so that the main signals which are not in the alarm state are supplied to the channel part and the overhead signal OH is supplied to the overhead input/output circuit 20.
Hence, the main signals and the overhead signal OH are positively transmitted and received via one of the working transmission line 3 and the protection transmission line 4.
On the other hand, when the working transmission line 3 and the protection transmission line 4 shown in FIG. 1 extend for a long distance, it becomes necessary to provide optical repeating installations 5 and 6 between the optical terminal apparatuses 1 and 2 for the respective working and protection transmission lines 3 and 4, as shown in FIG. 3.
FIG. 4 shows an example of the construction of the optical repeating installation 5 or 6 shown in FIG. 3. For the sake of convenience, it is assumed that FIG. 4 shows the construction of the optical repeating installation 5 which is inserted in the working transmission line 3.
In the optical repeating installation 5 shown in FIG. 4, a demultiplexer 32 and a multiplexer 31 are connected in series in a direction from the optical terminal apparatus 1 to the optical terminal apparatus 2. On the other hand, a demultiplexer 34 and a multiplexer 33 are connected in series in a direction from the optical terminal apparatus 2 to the optical terminal apparatus 1.
Identifier Circuits 35 and 36 decode an identifier (ID) of the apparatus which is the subject of the supervision and is included in the overhead signal OH that is separated from the main signals in the corresponding demultiplexers 32 and 34.
When the identifying circuit 36 judges from the overhead signal OH that the identifier does not identify the apparatus to which this identifying circuit 36 belongs, a logical sum circuit 37 inputs the overhead signal OH and supplies this overhead signal OH to the multiplexer 33. Similarly, when the identifying circuit 35 judges from the overhead signal OH that the identifier does not identify the apparatus to which this identifying circuit 35 belongs, a logical sum circuit 38 inputs the overhead signal OH and supplies this overhead signal OH to the multiplexer 31.
An overhead input/output circuit 39 inputs the overhead signal OH when the identifying circuit 35 judges from the overhead signal OH that the identifier identifies the apparatus to which this identifying circuit 35 belongs. In this case, the overhead input/output circuit 39 supplies a response signal to the logical sum circuit 37 in response to the overhead signal OH from the identifying circuit 35, so that the overhead signal OH is multiplexed with the main signals in the multiplexer 33 and returned to the transmission line 3 from which the overhead signal OH was received.
The overhead input/output circuit 39 also inputs the overhead signal OH when the identifying circuit 36 judges from the overhead signal OH that the identifier identifies the apparatus to which this identifying circuit 36 belongs. In this case, the overhead input/output circuit 39 supplies a response signal to the logical sum circuit 38 in response to the overhead signal OH from the identifying circuit 36, so that the overhead signal OH is multiplexed with the main signals in the multiplexer 31 and returned to the transmission line 3 from which the overhead signal OH was received.
In the optical communication system having the optical repeating installations 5 and 6 inserted in the transmission lines 3 and 4, the overhead signal OH is transmitted from the optical terminal apparatus 1 to the optical terminal apparatus 2 via the optical repeating installation 5 when the switching circuits 15 of the optical terminal apparatuses 1 and 2 select the working transmission line 3, and the overhead signal OH will not be transmitted further. In addition, the overhead signal OH from the optical terminal apparatus 2 is transmitted via the same working transmission line 3 and reaches the optical terminal apparatus 1 via the optical repeating installation 5.
Accordingly, when the communication is being made via the working transmission line 3 as described above, the optical repeating installation 6 in the protection transmission line 4 is cut off from the communication with the optical terminal apparatuses 1 and 2. In other words, the optical repeating installation 6 is disconnected from the network. As a result, the optical repeating installation 6 cannot obtain the overhead signal OH from the optical terminal apparatus 1 or 2, and there was a problem in that the optical repeating installation 6 cannot carry out the supervision of the alarm or the like.